Conventional microwave heating devices of this type have been structured to include a heating chamber having a rectangular parallelepiped shape, in general, wherein the heating chamber includes one or more radiation portions. Such plural radiation portions have been structured such that the radiation portions are provided on an upper wall surface and a bottom wall surface of the heating chamber and, also, the respective radiation portions are supplied with microwaves from dedicated microwave generating means. In other cases, such plural radiation portions have been structured such that two radiation portions are provided on side wall surfaces of the heating chamber and, also, the two radiation portions are supplied with microwaves from a single microwave generating means through a waveguide (refer to Patent Literature 1, for example).
Further, in some structures, plural radiation portions are dispersively placed on wall surfaces of a heating chamber, and microwave generating means are provided for supplying microwaves to the respective radiation portions, wherein, out of these microwave generating means, the microwave generating means placed on at least two wall surfaces are operated in a time-division manner (refer to Patent Literature 2, for example).
As described above, with the microwave heating device disclosed in Patent Literature 2, the selected microwave generating means are operated in a time-division manner, in order to prevent the microwave generating means connected to the radiation portions from being broken by microwaves received by these radiation portions due to interference of microwaves within the space in the heating chamber, which enables operating the plural microwave generating means substantially at the same time.
Further, by properly selecting connections between the heating chamber and the microwave generating means, for the radiation portions placed on the wall surfaces orthogonal to each other in the heating chamber, it is possible to suppress interference of microwaves radiated from both the radiation portions, thereby enabling oscillating the microwave generating means at the same time.
Some conventional microwave heating devices have been structured to include plural radiation portions and have been adapted to change the amounts of microwave electric power supplied to the respective radiation portions, through control of a phase shifter provided in a microwave generating means (refer to Patent Literature 3, for example).
In such a conventional microwave heating device, the microwave generating means includes an oscillation portion constituted by a semiconductor device, a dividing portion for dividing the output of the oscillation portion into plural parts, plural amplification portions for amplifying the respective outputs resulted from the division, a synthesis portion for synthesizing the outputs from the amplification portions, and a phase shifter provided between the dividing portion and the amplification portions. In such conventional microwave heating devices, the respective radiation portions for radiating microwaves within the heating chamber are connected to two outputs of the synthesis portion.
The phase shifter is structured to change over the microwave path line length by utilizing ON/OFF characteristics of diodes. Further, the synthesis portion is constituted by a 90-degree hybrid coupler or 180-degree hybrid coupler. By controlling the phase shifter, the electric power ratio between the two outputs from the synthesis portion is changed, or the phases of the two outputs are changed to be the same phase or opposite phases.
Further, some conventional microwave heating devices have been structured to radiate circularly polarized waves from radiation portions, in order to facilitate uniformization of heating of objects to be heated within a heating chamber (refer to Patent Literature 4, for example). Patent Literature 4 discloses a microwave heating device including a heating chamber which is provided, in a wall surface thereof, with a pair of opening portions orthogonal to each other, in order to enable radiations of circularly polarized waves.